Nonsag helical tungsten filament and method of making it



Patented Apr. 18, 1933 UNITED STATES" PATENT OFFICE JOEANNES ANTONIUS MARIA VAN LIEMPT- AND WILLY GEISS, OF EINDHOVEN, NETHERLANDS, ASSIGNORS TO GENERAL ELECTRIC COMPANY, A. CORPORATION OF NEW YORK NONSAG HELIGAL T'UNGSTEN FILAMENT AN D METHOD OF MAKING- IT No Drawing. Application flled April 27, 1925, Serial No. 26,290, and in the Netherlands May 24, 1924.

The invention relates to a method of manufacturing non-sagging helically coiled filaments made of metal with a high melting point, such as tungsten. In the incandescent lamp industry it is customary to wind drawn tungsten wire into a helix on a core, such as a steel wire, and after the winding is completed to remove this core by chemical means, for instance, by means of hydrochloric acid. The helically wound filaments obtained in this wa are then mounted by being hooked on the filament supports, and are then placed inside the bulb. Incandescent lamps filled with inert gas and having tungsten filaments made and mounted in this way have the drawback when operated at usual commercial efiiciencies that the coiled filaments.

strung between'two successive supports will sag somewhat when the lamp is used. This sagging stretches the helical windings, and

separates the turns of the coils, thus decreasing the efliciency of the lamp.

The object of this invention is to provide a method that eliminates this drawback of the coil sagging, and makes feasible the manufacture of helically coiled filaments that do not sag, even after having been in use for a long time, and that retain the original shap e of coil and spacing of the turns. Ex-

periments show that the sagging of the 'coiled filaments of drawn tungsten wire that have heretofore been used in incandescent lamps is due to a process of re-crystallization which takes place for the greatest part during the first few hours that the lamp burns and eventually produces in the metal a crystalline structure which prevents further sagging. By this invention a method is provided which almost or entirely completes the. process of recrystallization that pro-.

duces the non-sag structure before the spiral filament is hooked on the supports and placed in the bulb, so there will be little or no sag after the lamp, is put in use. According to this invention the manufacture of non-sagging coiled filaments of metal with a highmelting poigt, such as tungsten, for use in incandescent amps and in thermionic and similar apparatus is carried out by a process of recrystallizing the filament so as to produce in it 'a sag resisting crystalline structure before it is mounted in the incandescent lamp, thermionic apparatus, etc. Tungsten which is drawn into wire and is so .prepared that it is capable of developing the sag resisting crystalline structure is preferred as material for the coiled filaments. The temperature of recrystallization of tungsten is about 1500 C.,therefore, according to the invention, the helically coiled filaments of tungsten are heated without distor-,

tion to a temperature of over 1500 C. until recrystallization occurs The best results according to the invention will be obtained when the drawn tungsten wire is woundv on a core of a material that will remain solid when heated up to a temperature above the temperature of recrystallization of the tungsten wire; then the helically coiled filament on the core is heated to a temperature above its temperature of recrystallization, after which the wire core is removed. It is obvious that the steel wire heretofore used as a core for coiled filaments cannot be used in this method, for such a core cannot be heated above 1500 C. without melting. However, the core may be made of molybd um, tantalum, zirconium, or of alloys of ese metals, or of any other material that has a high enough melting point. Of course it is a necessary condition that the core can be removed from the helically coiled filament preferably by means of a chemical treatment. If the wire that serves as a core consists of molybdenum, or of an alloy of molybdenum with not too much tungsten, then according to the invention after heating the helical filament to the necessary tempera;

ture the core can be removed by means o a mlxture of concentrated sulphuric acid 5 one that occurs most often in the filament.

and'strong nitric acid, or by means of sele- V nium oxychloride. If the core consists of .la previously known methods were found to sag less as the structure of the crystals is coarser, that is, as the average size of the crystals is larger. Therefore, the sag of the helical filaments made according to the method that is described here will also depend on the average size of the crystals and on how they are joined or interlocked. If

' adefinite length of the filament is taken,

and all the crystals in that length are count-' ed lengthwise of the filament, and the total number of each size obtained, there can be constructed a curve which will show that a definite size of crystals occurs with maximum frequency and this size of crystals is the For brevitys sake in this description and in the cl alms', this size of crystal will be called the most probable size of the crystals.

term lamp will be used to mean not only lncandescent lamps, but also thermionic ap- I paratus, such as X-ray tubes, pliotrons, dynatrons,'kenetrons such as used in the radio .telegraphy and radiotelephony, and similar apparatus that contains a filament. Also in this description and in the claims the sag of a coiled filament will bemeasured in terms of the height of the segment of the circle of which the sagged coiled filament formspart of the circumference expressed as a percent-- age of the chord of the segment of filament will be understood from the following pr'ac The drawn tungsten wire suspended between adjacent supports.

Lamps made according to the invention contain a drawn helical filament consisting.

of crystals of which the most probable size is somewhat less than 0.05 millimetenandwhich after recrystallization are so over-. lapped and interlocked that the filament shows at the operating temperature of the lamp a sag of less than 10%, also a drawn tungsten helical filament consisting of or tals of which the most probable size lies tween 0.05 and 1 millimeter and which after recrystallization are so interlocked that the filament shows at the operating temperature of the lamps a sag of less than 5%.

The method of carrying out the invention tical example. capable of developing the non-sag structure is wound upon a core of molybdenum wire in the form of a helix by the usual coiling machine. Then the coiled filament and the core are placed in a region of high temperature,over-1500 C. At the same time, the

heated filament and core must be in an atmosphere of indifierent gases, such as a mixture of hydrogen and nitrogen. Then the helical filament, still wound on the core, is' cut to the length required, as for instance, for the making of incandescent lamps of a certain kind. Then the pieces of helical filament and core are laid'in a mixture of equal parts of concentrated sulphuric acid and strong nitric acid of which the specific gravity for instance is A or a solution of selenium oxychloride which chemically dissolves the molybdenum core and leaves the tungsten filament unchanged. A wire of tantalum or of zirconium may be used as a core in the same way as molybdenum and removed by means of hydrofluoric acid. A tungsten helical filament obtained in this way when cleaned and mounted in the bulb-will keep the original form it had when the bulb was sealed, even after long use in a gas filled lamp ments made according to this invention have a long life and their eificiency decreases less than that of lamps with filaments of similar In this description and in the claims the by Letters Patent of the United States, is,

1. The method of producing a helical tungsten filament which is substantially non-sag when mounted and used as a filament in a lamp in the usual way, which consists in winding drawn tungsten wire capable of developing non-sag crystalline structure into a helix on a core which is solid at temperatures above the recrystallizing temperature of tungsten, heating the core with the coil on .it above the temperature of recrystallization to develop said non-sag structure, and then chemically removing the core by re-agents inert to tungsten.

2. The method of producing a helical tungsten filament which is substantially non-sag when mounted and used as a filament in a lamp in the usual way, which consists in winding drawn tungsten wire capable of de- 'veloping. non-sag crystalline structure into a helix on a wire mandrel of metal melting at a temperature above 1800 C. and remov-' able by chemical reagents inert to tungsten, heating the mandrel and coil above 1500 C. to develop in said tungsten the non-sag structure, and then rem ving the mandrel with chemical reagents w ich do not attack tungsten.

3. The method of producing a helical tungsten filament which is substantially non-sag when mounted and used as a filament in an electric incandescent lamp, which consists in coiling drawn tungsten wire capable of developing non-sag crystalline structure on a mandrel substantially circular in cross secrecrystallization and then tion which has a melting point above a temperature of recrystallizatmn of tungsten to develop non-sag structure,

heating the coil and mandrel above the said temperature of chemically removing the mandrel by a reagent inert to tung-, sten In witness whereof,

vve have hereunto set our hands this 10th day of April, 1925.

' JOHANNES ANTON! WILLY GEISS.

Us MARIA vAn uulr'r. 

